Flexible insert for heat pipe freeze protection

ABSTRACT

A flexible, pressurized insert for insertion within the hot (evaporator) side of an inclined heat pipe that forms a part of a heat pipe heat exchanger. This insert is employed in order to prevent such heat pipe from rupturing in the event the working fluid contained therein freezes. This insert is constructed of a thin-walled flexible material that is capable of being deformed (i.e. compressed) thereby absorbing the expansion pressures exerted by the working fluid should the working fluid freeze or be exposed to freezing temperatures. By such absorption, the outer heat pipe itself will not be over-pressurized which might otherwise lead to its rupture. Upon the thawing of the working fluid, the pressurized insert (which is at a pressure greater than that of the working fluid in its non-frozen state) will once again regain its shape.

FIELD OF THE INVENTION

This invention pertains to heat pipe heat exchangers in general and moreparticularly to a manner of preventing the heat pipe from rupturing inthe event the working fluid therein freezes.

DESCRIPTION OF THE PRIOR ART

Heat pipe heat exchangers are well known and are commonly used to heatand cool both gases and liquids. They operate by passing hot fluidthrough an array of heat pipes which contain a working fluid therein.This working fluid acts as the heat transport medium which absorbs theheat from the hot side of the heat exchanger and transports such heat tothe cold side where it is cooled. Upon such cooling, the working fluidagain becomes available for heat absorption from the hot side of theheat exchanger. This cycle then repeats itself.

One major problem associated with heat pipe heat exchangers is thefreezing of the working fluid when the heat exchanger is not inoperation. Such freezing causes this working fluid (generally water) toexpand thereby potentially rupturing the heat pipe causing acatastrophic failure of the heat exchanger, or at least a portionthereof.

In the past, three general methods have surfaced to address thisproblem. First, the wall thickness of the heat pipe was increased so asto withstand the forces imposed by the freezing water. However, as canbe imagined, this greatly increases the cost of the heat exchanger.

Second, chemical additives were added to the working fluid to lower thefreezing temperature of the working fluid below the lowest expectedoperating temperature. Unfortunately, such chemical additives are oftenhazardous and they may not be able to suppress the freezing pointsufficiently for some applications in which the exposure or ambienttemperature is considerably below the adjusted freezing point. Also,over time, such chemicals have a tendency to break down thereby reducingtheir ability to lower the freezing point as needed (such chemicalbreakdown being unknown to the operator until such time that a heat piperuptures). Furthermore, the use of chemicals in the working fluidresults in a coating on the inside wall of the heat pipe therebyblanketing the heat pipe and reducing its effectiveness. Chemicaladditives may also cause corrosion on the heat pipe wall surfaces andthey may adversely react with the working fluid or the gases containedtherein.

The third method employs a controlled heat source, such as an electricheater, to maintain the working fluid in the heat pipe above freezingtemperatures. Such external heaters significantly increase the cost ofthe heat exchanger since they add complexity and must be protected fromthe elements while still being able to supply heat to the exposed heatpipe. Also, such heaters do not provide protection during transportationwhen power is not available or during instances of power failure (whichusually occur as a result of the sudden onslaught of cold and icytemperatures).

It is thus an object of this invention to provide another manner ofpreventing damage to the heat pipe as a result of the freezing of theworking fluid in heat pipe heat exchangers. Another object of thisinvention is to provide such freeze protection without the need forpower or external connections so that freeze protection is alwaysavailable even during transportation and construction of the heatexchanger, during power outages, and when the heat exchanger is takenout of operation such as when being repaired or maintained. A furtherobject of this invention is to provide freeze protection that will notinterfere with the heat exchange operation occurring in the heat pipe.Yet another object of this invention is to provide freeze protectionthat will not break down nor undergo a reduction in its ability toprotect over time. Still another object of this invention is to providesuch freeze protection at low cost and with only marginally increasedmanufacturing and material costs. These and other objects and advantagesof this invention will become obvious upon further investigation.

SUMMARY OF THE INVENTION

This invention pertains to a manner of protecting inclined heat pipes,such as those normally found within a heat pipe heat exchanger, fromrupturing in the event the working fluid contained therein freezes. Itconsists of generally centrally locating an insert within the hot sideof the heat pipe where the working fluid normally collects. This insertis immersed within this working fluid and extends up to or slightlybeyond the at-rest level of the working fluid in the heat pipe. Agas/liquid mixture is contained within the insert with this mixturebeing at a pressure greater than the pressure of the non-frozen workingfluid (i.e. the pressure normally found in the heat pipe). This insertis generally constructed of a thin-walled material or foil pillow thatcan flex and be deformed without failing. Consequently, as the workingfluid expands while it freezes, the insert contained therein iscompressed by the greater forces exerted by the frozen working fluid.Such compression of the insert thus avoids any over-pressurization orfailure of the heat pipe due to the expansion of the working fluid. Thiscompression of the insert is accomplished by the further pressurizationof the gas/liquid mixture contained therein. Upon the thawing of theworking fluid, the pressurized insert flexes back to its normal shapedue to the now greater pressure in the insert over that of thenon-frozen working fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pictorial view of a typical heat pipe of a heat pipe heatexchanger with the invention shown therein.

FIG. 2 is a pictorial sectional view taken along lines 2--2 of FIG. 1and illustrating the location and operation of the invention within theheat pipe.

FIG. 3a-c are pictorial sectional views similar to that of FIG. 2 butillustrating different embodiments or configurations of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring initially to FIG. 1, there is shown a pictorial view of a heatpipe 10 normally found within a heat pipe heat exchanger 12. Such heatexchanger 12 normally incorporates a divider plate 14 which separateshot or heat absorption side 16 from cold or heat rejection side 18. Asindicated, heat pipe 10 extends across such sides 16 and 18 at a slightincline, this incline rising from hot side 16 toward cold side 18. Thepurpose of this incline of heat pipe 10 is so that working fluid 20(usually water, but it can also be methanol, ammonia or the like) willnaturally gravitate or collect in hot side 16 of heat exchanger 12.Consequently, should heat exchanger 12 not be operating (such as whenbeing repaired, maintained or constructed), the collected working fluid20 in hot side 16 is subject to freezing should such fluid 20 be exposedto freezing temperatures.

To prevent heat pipe damage from such freezing, a flexible insert 22 isplaced in hot side 16 of heat pipe 10. This insert 22 would ideally becentrally located within heat pipe 10 and would extend along the wettedlength of the working fluid 20 that collects within inclined heat pipe10 as shown. Insert 22 will also preferably extend to or slightly abovethe at-rest level 24 of working fluid 20 as indicated.

FIG. 2 illustrates such a central position of insert 22 within heat pipe10. This figure also indicates that insert 22 is a thin-walled tube,such as a foil pillow, having an hour-glass shape. Insert 22 is alsofully sealed on all its sides and ends. Other possible configurations ofinsert 22 are shown in FIG. 3. Of course, insert 22 can be any shapedesired and need not be limited to solely those shapes disclosed herein.The important feature of insert 22 is that it should be constructed of asealed, thin, impermeable, easily deformed material, such as a metalfoil or the like.

No matter what its configuration, insert 22 is filled with a smallamount of liquid 26 which is generally the same as working fluid 20. Apressurized inert gas 28 fills the remaining greater volume of insert 22thereby pressurizing insert 22 to a greater extent than the remainder ofheat pipe 10 (i.e. a positive pressure differential exists across insert22). In other words, the internal pressurization of insert 22 is greaterthan the external forces acting upon it when working fluid 20 is notfrozen. However, when working fluid 20 freezes, such fluid 20 willexpand thereby generating freezing pressures greater that the internalpressure of insert 22. Consequently, rather than causing heat pipe 10 torupture, such freezing/expansion pressures of working fluid 20 areeasily accommodated by the compression of insert 22.

One purpose of such a positive pressurization within insert 22 whenworking fluid 20 is not frozen is so that insert 22 will `spring back`to its original shape once the freezing pressures acting upon it are nolonger present or are reduced. If insert 22 were not soover-pressurized, insert 22 would most likely retain its deformed stateafter the first freeze cycle and thus would not provide the neededprotection (or `give`) required for subsequent freeze cycles.

The cross-section of insert 22 is shaped so as to allow for the flexingof insert 22 under the external freezing pressures exerted upon itduring the freezing and expansion of working fluid 20. This shape isconfigured so as to allow plastic and/or elastic deformation or flexingwithout failing or rupturing. Consequently, insert 22 will flex andcompress during the freezing (i.e. expansion) of working fluid 20 sothat outer heat pipe 10 will not be subject to such forces and thuspossibly fail. Alternatively, insert 10 will absorb a sufficient amountof the generated freezing pressures such that any remaining pressurewill not be sufficient enough to cause damage to outer heat pipe 10. Thehour-glass configuration of insert 22 is designed to contract at its`waist` section so as to accommodate any external ice pressure it mightbe subject to. Such an hour-glass shape could also result from plasticdeformation after the first freeze cycle of an oval shaped insert 22 asshown in FIG. 3a.

As stated earlier, insert 22 will only extend within heat pipe 10 withinhot end 16 so as to be immersed within working fluid 20. It serves nopurpose for insert 22 to extend along the full length of heat pipe 10since freeze protection is only required where working fluid 20collects. During any freezing of working fluid 20, the expansion of suchworking fluid 20 will cause insert 22 to be compressed. This willprevent any build up of any freezing pressures against the walls of heatpipe 10 thereby eliminating any possibility of such heat pipe 10rupturing due to the freezing of working fluid 20. Instead, suchfreezing pressures will be accommodated by insert 22. However, onceworking fluid 20 thaws, the over-pressurization of insert 22 will returninsert 22 to it original shape, ready for the next onslaught of freezingpressures.

While FIGS. 3a-3c disclose additional configurations of insert 22 (FIG.3a being an oval, FIG. 3b being a cross, and FIG. 3c being a tear-drop),other configurations are also likely. The important features of anyshaped insert 22 are as follows:

(a) its construction of a sealed, impermeable, flexible, thin-walledmaterial such as a foil pillow or the like,

(b) its over-pressurization with respect to the pressure of workingfluid 20 (i.e. the interior of heat pipe 10) in its non-frozen state,

(c) the ability of insert 22 to flex and give when working fluid 20freezes, yet return to its original shape once the threat of freezing isno longer present,

(d) the containment of a small amount of liquid 26 (generally the sameas working fluid 20) within insert 22 with the remainder of insert 22being filled with an inert pressurized gas, and

(e) extending insert 22 only within the working fluid 20 region of hotside 16 of the heat pipe 10.

Such insert 22 is retained in its generally central position within heatpipe 10 by the use of straps or supports (not shown) which supportinsert 22 along its length. Preferably, such straps or inserts will notbe continuous but instead will be intermittent or spaced along thelength of insert 22 so as not to interfere with the flow or movement ofworking fluid 20 within heat pipe 10. The actual construction of insert22 may be thin carbon or stainless steels, but other materials are alsolikely so long as they are strong and flexible enough to withstandrepeated applications of freezing pressures (i.e. repeated deformations)and they do not react with either working fluid 20 or heat pipe 10.

While the above is described with respect to a heat pipe heat exchanger12, it should be readily understood that insert 22 is equally applicableto any liquid containing pipe or conduit which is subject to freezingand/or rupture if exposed to freezing temperatures.

What is claimed is:
 1. An insert for freeze protecting an elongated pipecontaining a working fluid therein comprising an elongated, sealed,thin-walled, flexible insert extending within the working fluid of thepipe and containing a liquid/gas mixture therein, said insert having aninternal pressure greater than that of the working fluid in itsnon-frozen state, said insert being compressed upon the freezing of theworking fluid thereby accommodating such expansion of the working fluidwithin the confines of the pipe without over-pressurizing the pipe, saidinsert expanding in volume when the freezing forces generated by theworking fluid recede.
 2. The insert as set forth in claim 1 wherein theliquid portion of said liquid/gas mixture within said insert is the sameas the working fluid and wherein the gas portion of said liquid/gasmixture within said insert is inert.
 3. The insert as set forth in claim2 wherein said insert is generally centrally located within the pipe andterminates at or slightly above the level of the working fluid in thepipe.
 4. The insert as set forth in claim 3 wherein the longitudinalaxis of said insert is generally parallel to the longitudinal axis ofthe pipe.
 5. The insert as set forth in claim 4 wherein saidlongitudinal axis of said insert is co-axial with the longitudinal axisof the pipe.
 6. The insert as set forth in claim 4 wherein said insertis constructed of a metal foil.
 7. The insert as set forth in claim 4wherein said working fluid comprises water or is water based.
 8. Afreeze protected heat pipe comprising:(a) an elongated, closed heat pipecontaining a working fluid therein, said heat pipe being at a firstpressure; and, (b) an elongated, closed, thin-walled, flexible insertextending within said working fluid of said heat pipe and containing aliquid and a gas therein, said insert being at a pressure greater thansaid first pressure, said insert being compressed upon the freezing ofthe working fluid thereby accommodating such expansion of the workingfluid within the confines of the pipe without over-pressurizing thepipe, and wherein said insert generally returns to its original shapeupon the thawing of the working fluid.
 9. The insert as set forth inclaim 8 wherein said liquid within said insert is the same as theworking fluid and wherein said gas within said insert is inert.
 10. Theinsert as set forth in claim 9 wherein said insert is generallycentrally located within said heat pipe and terminates at or slightlyabove the level of said working fluid in said heat pipe.
 11. The insertas set forth in claim 10 wherein the longitudinal axis of said insert isgenerally parallel to the longitudinal axis of the pipe.
 12. The insertas set forth in claim 11 wherein said longitudinal axis of said insertis co-axial with the longitudinal axis of the pipe.
 13. The insert asset forth in claim 11 wherein said insert is constructed of a metalfoil.
 14. The insert as set forth in claim 11 wherein said working fluidcomprises water or is water based.
 15. A freeze protected heat pipe heatexchanger comprising:(a) a plurality of inclined elongated heat pipeextending from a lower hot side of the heat exchanger to an upper coldside thereof, said heat pipe containing a working fluid in said hot sideat a first pressure; and (b) an elongated, closed, thin-walled, flexibleinsert immersed within said working fluid in said hot side of said heatpipe, said insert containing a liquid and a gas therein at a pressuregreater than said first pressure, said insert being compressed upon thefreezing of the working fluid thereby accommodating such expansion ofthe working fluid within the confines of the pipe withoutover-pressurizing the pipe, and wherein said insert generally returns toits original shape upon the thawing of the working fluid.
 16. The insertas set forth in claim 15 wherein said liquid within said insert is thesame as the working fluid and wherein said gas within said insert isinert.
 17. The insert as set forth in claim 16 wherein said insert isgenerally centrally located within said heat pipe and terminates at orslightly above the level of said working fluid in said inclined heatpipe.
 18. The insert as set forth in claim 17 wherein the longitudinalaxis of said insert is generally parallel to the longitudinal axis ofthe pipe.
 19. The insert as set forth in claim 18 wherein saidlongitudinal axis of said insert is co-axial with the longitudinal axisof the pipe.
 20. The insert as set forth in claim 18 wherein said insertis constructed of a metal foil.
 21. The insert as set forth in claim 18wherein said working fluid comprises water or is water based.